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Parkinson's disease, depression linked?

While fans and friends have looked for answers to why the 63-year-old comedy icon would take his own life, his wife, Susan Schneider, issued a written statement about Williams' health that he had kept a secret.

"Since his passing, all of us who loved Robin have found some solace in the tremendous outpouring of affection and admiration for him from the millions of people whose lives he touched," Schneider said. "His greatest legacy, besides his three children, is the joy and happiness he offered to others, particularly to those fighting personal battles.

The best Robin Williams tributes

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In his own words: Williams on depression

"Robin's sobriety was intact and he was brave as he struggled with his own battles of depression, anxiety as well as early stages of Parkinson's disease, which he was not yet ready to share publicly.

Photos: Robin Williams' career 28 photos

Photos: Robin Williams' career28 photos

Comedic actor Robin Williams dies – Robin Williams was honored during 2014's Emmy telecast with a tribute led by friend Billy Crystal, who hosted the "Comic Relief" benefits with Williams and Whoopi Goldberg (seen here in 1986). Williams died August 11 at age 63. Click through to see moments from the beloved actor's remarkable life.

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Photos: Robin Williams' career28 photos

Comedic actor Robin Williams dies – Williams' peers regarded him as a brilliant actor and comedian. His friend David Letterman remembered him as "nothing we had ever seen before."

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Photos: Robin Williams' career28 photos

Comedic actor Robin Williams dies – Williams first shot to stardom with Pam Dawber in the sitcom "Mork & Mindy" in September 1978.

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Photos: Robin Williams' career28 photos

Comedic actor Robin Williams dies – Williams attends the Robin Williams Opening Party on April 11, 1979, at Studio 54 in New York City.

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Photos: Robin Williams' career28 photos

Comedic actor Robin Williams dies – This 1982 file photo originally released by Warner Bros. Pictures shows Williams as T.S. Garp from the film "The World According to Garp."

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Photos: Robin Williams' career28 photos

Comedic actor Robin Williams dies – Williams and his first wife, Valerie Velardi, join a huge number of photographers packed into singer Paul Simon's apartment to celebrate Simon's wedding to actress Carrie Fisher in New York City on August 16, 1983.

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Photos: Robin Williams' career28 photos

Comedic actor Robin Williams dies – Williams, center, takes time out from rehearsal at NBC's "Saturday Night Live" with cast members Eddie Murphy, left, and Joe Piscopo on February 10, 1984. Williams would appear as guest host on the show.

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Photos: Robin Williams' career28 photos

Comedic actor Robin Williams dies – Williams enjoys music through a headset in a scene from the film "Good Morning, Vietnam" in 1987.

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Photos: Robin Williams' career28 photos

Comedic actor Robin Williams dies – Williams portrayed a teacher in the movie "Dead Poets Society" in 1989, one of his first mostly dramatic roles.

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Photos: Robin Williams' career28 photos

Comedic actor Robin Williams dies – Williams went to all lengths to stay with his children in the 1993 movie "Mrs. Doubtfire."

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Photos: Robin Williams' career28 photos

Comedic actor Robin Williams dies – From left, Bonnie Hunt, Bradley Pierce, Kirsten Dunst and Williams hold one another in a scene from the 1995 film "Jumanji."

Comedic actor Robin Williams dies – Jay Leno laughs as Williams jokes around during a taping of "The Tonight Show with Jay Leno" on November 13, 1995, at the MGM Grand Hotel in Las Vegas.

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Photos: Robin Williams' career28 photos

Comedic actor Robin Williams dies – Williams and Nathan Lane starred in the film "The Birdcage" in 1996.

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Photos: Robin Williams' career28 photos

Comedic actor Robin Williams dies – Williams visits a sick child in a scene from the film "Patch Adams" in 1998.

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Photos: Robin Williams' career28 photos

Comedic actor Robin Williams dies – Williams and his wife, Marsha, pose for photographers with their daughter, Zelda, as they arrive at the premiere of the film "Patch Adams" in December 1998 in New York City.

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Photos: Robin Williams' career28 photos

Comedic actor Robin Williams dies – Williams wears a clown nose as he places his hands in concrete during a ceremony outside Mann's Chinese Theatre in Hollywood in 1998. In the critically acclaimed "Patch Adams," he played a doctor who used humor to help heal his patients

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Photos: Robin Williams' career28 photos

Comedic actor Robin Williams dies – Actor-writers Matt Damon, left, and Ben Affleck, right, pose with Williams, holding the Oscars they won for "Good Will Hunting" at the 70th annual Academy Awards in 1998. Damon and Affleck won for best original screenplay, and Williams won for best supporting actor.

Comedic actor Robin Williams dies – U.S. Postal Service team rider Lance Armstrong rides with Williams during training on a rest day of the 89th Tour de France cycling race in Vaison La Romaine on July 22, 2002.

Comedic actor Robin Williams dies – Williams at the pre-premiere party for "Happy Feet Two" at the Grand Connaught Rooms in London on November 20, 2011.

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Comedic actor Robin Williams dies – Williams poses for photographs in Sydney, Australia, on December 5, 2011. He and Australian director George Miller were in Australia to promote "Happy Feet Two."

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Comedic actor Robin Williams dies – Upon his death, Williams' wife, Susan Schneider, said, "This morning, I lost my husband and my best friend, while the world lost one of its most beloved artists and beautiful human beings. I am utterly heartbroken."

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EXPAND GALLERY

"It is our hope in the wake of Robin's tragic passing, that others will find the strength to seek the care and support they need to treat whatever battles they are facing so they may feel less afraid."

It is not clear whether the early-stage Parkinson's disease affected his ability to work.

"Friends and family can usually detect changes in the Parkinson's patient including poor posture, loss of balance, and abnormal facial expressions," according to the National Parkinson Foundation. "During this initial phase of the disease, a patient usually experiences mild symptoms. These symptoms may inconvenience the day-to-day tasks the patient would otherwise complete with ease. Typically these symptoms will include the presence of tremors or experiencing shaking in one of the limbs."

Williams used exercise and cycling to manage his stress and depression, and the prospect that the illness would prevent him from doing that was extremely upsetting, adding to the depression, the person familiar with his family said.

Fellow actor Michael J. Fox, who has Parkinson's and established the Michael J. Fox Foundation, said Thursday that he was unaware of his friend's condition.

"Stunned to learn Robin had PD. Pretty sure his support for our Fdn predated his diagnosis. A true friend; I wish him peace," Fox tweeted.

Investigators believe Williams used a belt to hang himself from a bedroom door sometime between late Sunday and when his personal assistant found him just before noon Monday, according to Marin County Assistant Deputy Chief Coroner Lt. Keith Boyd.

Boyd would not confirm or deny whether Williams left behind a letter, saying that investigators would discuss "the note or a note" later.

The coroner's investigation "revealed he had been seeking treatment for depression," Boyd said.

He spent time in a treatment facility in July, a time when his wife and representative have said he was battling depression.

Media reports at the time speculated that Williams had resumed drinking alcohol, but the statement from his wife appears to dispute those reports.

Williams entered rehab because of drug and alcohol addiction at least twice previously.

"Robin spent so much of his life helping others," his wife said. Whether he was entertaining millions on stage, film or television, our troops on the front lines, or comforting a sick child -- Robin wanted us to laugh and to feel less afraid."